1. Field of the Invention
The present invention relates to an ion trap mass spectrometer and an ion trap mass spectrometry method in which ions obtained by ionizing a sample are captured in an ion trap, and the ions are dissociated and subjected to mass spectrometry to perform MSn analysis (n is an integer of 2 or greater).
2. Description of the Related Art
An ion trap mass spectrometer provided with an ion trap is widely used in identification of a high-molecular compound such as a peptide from a mixture sample such as a biological sample (see, for example, Andrew N. Krutchinsky, Markus Kalkum, and Brian T. Chait, Automatic Identification of Proteins with a MALDI-Quadrupole Ion Trap Mass Spectrometer, Anal. Chem., 2001, 73(21), 5066-5077). In this type of mass spectrometer, for example, a sample is vaporized in vacuum together with a matrix by MALDI (matrix-assisted laser desorption-ionization), and the sample is ionized by delivery of protons between the sample and the matrix. Ions obtained by ionizing the sample can be then captured in an ion trap and subjected to mass spectrometry.
FIG. 9 is a flow chart showing one example of process when mass spectrometry is performed by a conventional ion trap mass spectrometer. In this example, ions captured in an ion trap are dissociated by so called CID (collision-induced dissociation) and subjected to mass spectrometry to perform MSn analysis.
First, mass spectrometry (MS1 analysis) of an ionized sample is performed to measure a MS1 spectrum (step S501). The MS1 spectrum is then analyzed to detect an ion corresponding to a peak that satisfies a predetermined criterion as a MS2 precursor ion (steps S502 and S503).
When the MS2 precursor ion is detected (Yes in step S504), ions obtained by ionizing the sample are captured in an ion trap, ions detected as MS2 precursor ions are left in the ion trap and dissociated one by one, and subjected to mass spectrometry (MS2 analysis) to measure a MS2 spectrum (step S505). Thereafter, the MS2 spectrum is analyzed to detect an ion corresponding to a peak that satisfies a predetermined criterion as a MS3 precursor ion (steps S506 and S503).
In this manner, MSn analysis is performed by repeating the processes in steps S503 to S506 until the MSn precursor ion (n is an integer of 2 or greater) is no longer detected (No in step S504). Sample components can be identified based on the MSn spectrum obtained by the MSn analysis.
In the conventional ion trap mass spectrometer described above, one MS2 precursor ion is usually selected from each peak and MS2 analysis is performed when the MS1 spectrum has a plurality of peaks that satisfy a predetermined criterion. That is, an attempt has not been made to identify a plurality of peptides by performing measurement for a plurality of MS2 precursor ions in parallel.
Therefore, every time MS2 analysis is performed for a MS2 precursor ion corresponding to each peak in a MS1 spectrum, a sample is ionized to reduce the amount thereof, so that the sample may be exhausted before all the components are identified. The measurement time is increased, so that a matrix may be sublimed in vacuum, thus making it impossible to continue measurement. Particularly, DHB (2,5-dihydroxybenzoic acid), a typical compound of a matrix is easily sublimed in vacuum.
The present invention has been devised in view of the above-described situations, and an object of the present invention is to provide an ion trap mass spectrometer and an ion trap mass spectrometry method which can realize reduction of the number of times that a sample is ionized, and shortening of the measurement time.